Measurements if no Agent is in the measurement area

Currently, some processors set the value to 0 if no agent is in the measurement area. However, this can have unwanted side effects (e.g. for speed 0 also means, that all agents in the area do not move -- also when one agent enters the area, the speed jumps from zero to free flow speed).

Therefore, I plan to set the value to nan in cases where no agent is present (i.e. invalid, no measurement can be done). This means, the user can individually decide how to deal with this case.

cc @hm-mgoedel @hm-kleinmei